This invention relates generally to gas turbine engines and more particularly to a turbofan engine in which the fan flow can be modulated independent of the fan speed.
It is known to extract bleed air from a turbine engine to perform functions such as flap blowing, boundary layer control, and lift enhancement in an aircraft. In particular, Short Takeoff and Landing (STOL) aircraft can utilize engine bleed air for wing lift enhancement during the take-off and landing phases of flight. Such aircraft require that the bleed air flow and pressure levels remain essentially constant, even though the engine thrust will vary over a band of about 20% to 100% of maximum, depending on the phase of flight. Bleed pressure levels must also be sufficiently high to keep pipe sizes reasonable for a given bleed energy level. Also, in a multi-engine aircraft, for one engine out operation, the engine system must be capable of generating the equivalent bleed energy of that needed with all engines operating. Immediate operating engine response to an engine out failure is also desired. These requirements present several problems for conventional engine systems since engine bleed air source pressures and flows vary widely over the operating thrust band and, during approach and landing, LP spool speeds are greatly reduced which can result in unacceptable spool-up times.